Many data storage systems, such as optical disk or tape, move the data storage media at variable speeds. In optical disk systems, such as CD-ROM or DVD-ROM, the disk is rotated so as to maintain a constant linear velocity (CLV) across the full recording surface, which comprises a single spiral track from the inner diameter to the outer diameter. Data at the inner tracks is written and read at the same linear velocity (in the track direction) as the outer tracks. However, since the location of the tracks are different distances from the hub, the rotation speed of the drive must be adjusted for the given distance of the reading head from the hub. This arrangement maximizes the data storage capacity of the disk as compared to a constant angular velocity, such as magnetic disk drives which store the same amount of data on each track at the same data rate regardless of the distance from the hub. The CLV arrangement is ideal for data having a continuous format, such as a motion picture.
Modified versions of constant linear velocity systems, and mixed constant linear velocity, constant angular velocity systems also exist. One example is DVD-RAM, which, in one option, is constant angular velocity within a zone, and the zones are stepped in frequency, and, in another option, is constant linear velocity within a zone with a discontinuity at a zone boundary to allow for spares.
However, random access between CLV tracks a considerable radial distance from each other, or across modified CLV and CLV/CAV zone boundaries, requires that data recovery channel must delay until the motor speed is sped up or slowed down and adjusted to provide the correct linear velocity. Random access is important, for example, in interactive games or for non-sequential data processing. The wait for motor speed adjustment may degrade the effective data rate of the device very substantially.
Tape drives also run at a constant linear velocity, typically, for the full length of the tape. Data storage tape drives capable of random access, however, are required to stop and start at specific records. Thus, the data recovery channel must also wait until the motor speed is adjusted to provide the correct linear velocity, which degrades the effective data rate of the tape drive.
Optical and tape data storage systems employ high linear density recording (closely spaced data) to give both high capacity and a high instantaneous data rate. Such systems, however, require a very precise data clocking system which operates at a sampling rate which is some multiple of the center data frequency. An example is a phase locked loop (PLL) which locks the sampling rate to the frequency of the data. The most precise phase locked loop is a digital PLL. The PLL is provided with the Tau, which relates the sample rate to the period of the nominal center frequency of the data. In order to track the data precisely and follow small variations in the data rates, the digital PLL will vary the clocking sample rate slightly, but does not vary substantially from the supplied Tau.
Thus, the precise nature of the PLL, and the ability to adapt rapidly to a highly variable data frequency are at odds.